A mixer for producing mixed rubber which is a material of a rubber product by mixing a material to be mixed with various additives blended with, for example, raw rubber by rotating a pair of mixing rotors in a casing is generally known.
An introduction port for introducing the material to be mixed is provided at a casing of the mixer. During mixing, the introduction port is blocked by a floating weight, and when introducing the material to be mixed through the introduction port, the floating weight is moved to a position at which it does not interfere with the introduction of the material to be mixed.
A mixer disclosed in PTL 1 is provided with a mechanism for moving a unit composed of a floating weight and an air cylinder which lifts and lowers the floating weight, in a horizontal direction, for the purpose of reducing the height of the mixer.